Adjacent faces are separated on the UV map
There's a problem in your model's topology.  Do this experiment.  In the UV editor, after you've done the cylinder projection, deselect everything and then carefully select the top row of vertices:

Since you've enabled UV Sync Selection, the same row of vertices shows up in the 3D view:

But if you zoom in to the active vertex you can see an issue:

It might be easier to see in edge select mode:

the path jumps from one edge loop to the other.  I don't know how to solve this directly, but my horrible work around is this:
select one of the vertical edge loops:

delete everything else:

add a screw modifier with the same number of steps:

and apply the screw modifier, in object mode.  The result is:

Notice that I didn't mark a seam, because I didn't know where you'd want it.
I don't honestly know what the topology issue is, but this &quot;fixes&quot; it.